New transforming murine type C viruses were isolated and characterized with respect to target cell specificity in vivo, ability to transform cells in culture and their genome structure. Findings included (a) differences in infectivity between leukemia viruses from low and high leukemia mouse strains were directly related to leukemogenicity, (b) the chromosome residence was identified for the receptor-gene code for various classes of viruses, including new oncogenic isolates, (c) a lung carcinoma-associated virus was molecularly cloned, heteroduplex mapping showed a close relationship to spleen focus forming virus (SFFV) in the 3' half of the genome, (d) an attenuated variant of NIH Cl 6, a rapid leukemia inducing murine leukemia virus (MuLV), was isolated after molecular cloning. The genome of this virus was found to be identical with the on cogenic parent MuLV except near the long terminal repeat (LTR), (e) nonproducer transformed cells were isolated after infection with newly derived histiocytoma-inducing virus. The proviral genome from this virus which does not contain the tumor gene of Kirsten sarcoma virus (KiSV) or Moloney sarcoma virus (MoMSV) is presently being purified by molecular cloning.